High Temperature Corrosion Measurement and Control
A refinery’s profitability depends in part on blending feedstock to include relatively inexpensive ‘acid’ or ‘opportunity’ crudes, often of high Total Acid Number (TAN), which are in part low priced because of the corrosivity of the high temperature liquid fraction distilled from them. Specifically, between 280 to 400 °C, naphthenic acid – in fact a variety of organic acids found in crude oil and concentrated in refinery crude distillation units – and acidic sulfur species corrode steel according to a complex mechanism of scale dissolution/erosion and corrosion, which is referred to generically as ‘NAC’. When corrosion occurs it promotes hydrogen through steel which is readily measured and monitored using Hydrosteel 6000 with AT-R or HT-R probes.
Naphthenic acid corrosion generally occurs at pipe bends and reducer sections. Hydrogen flux monitoring has been demonstrated to be effective in identifying the corrosivity of naphthenic acid on a day by day basis (J.M.O’Kane, T.F.Rudd, D.Cooke, F.W.H.Dean, S.J.Powell, ‘Detection and monitoring of naphthenic acid corrosion in a visbreaker unit using hydrogen flux measurements’, Corrosion 2010, Paper 10351, Conf. series, NACE, Houston, 2010). There appears to be fairly good correlation of hydrogen flux with naphthenic acid corrosion.